Growth of Ta2SnO6Films, a Candidate Wide-Band-Gap p-Type Oxide

In an effort to discover a high-mobility p-type oxide, recent computational studies have focused on Sn2+-based ternary oxides. Ta2SnO6has been suggested as a potentially useful p-type material based on the prediction of simultaneously high hole mobility and a wide range of synthesis conditions over which it is the energetically favored phase. In this study, we synthesized this material epitaxially for the first time and evaluated its properties experimentally. We measured the band gap to be 2.4 eV and attempted to substitutionally dope titanium for tantalum (TiTa’) and potassium for tin (KSn’) but found that both doped and undoped films were insulating. Amorphous Ta2SnO6films were also grown via thermal atomic layer deposition (ALD) at 175 °C. Electrical characterization of the ALD-fabricated amorphous films found them to be insulating with an optical band gap of 2.24 eV. Density functional theory calculations indicate that, under MBE growth conditions, oxygen vacancies have a negative energy of formation in crystalline Ta2SnO6when the Fermi energy lies near the valence band edge. These oxygen vacancies would lead to compensation of holes generated by TiTa’or KSn’dopants, which is consistent with our observations. We conclude that the direct growth of epitaxial p-type Ta2SnO6films using MBE-accessible growth conditions is thwarted by the spontaneous formation of oxygen vacancies. While our growth conditions do not yield p-type films, we calculate that there are conditions under which Ta2SnO6is the thermodynamically stable phase and spontaneous formation of compensating defects does not occur, motivating further studies with different synthesis techniques.